In chemical experiments, during the process of the external operations, often biochemical substances that are harmful to humans are produced. Fume hoods are used as one type of equipment to prevent these biochemical substances from being diffused into a room and to prevent them from coming into contact with the human body. Typically, fume hoods are provided with an enclosure with a sash that can be opened either vertically or horizontally, where an operator in the laboratory can access the inside of the enclosure through the sash. So that the operator will not be exposed to harmful biochemical substances during the operations using the fume hood, the enclosure is connected to a local exhaust duct that removes the biochemical substances.
The room pressure controlling system is a system for maintaining a constant pressure within a room so that when biochemical substances are handled within a fume hood, the airflow rate of the local exhaust duct will be adjusted so as to maintain a specific speed for the planar airflow rate within the sash plane so that there will be no backflow of biochemicals into the room, so that biochemical substances will not leak out of the room and that contamination, and like, will not leak into the room (See, for example, Japanese Unexamined Patent Application Publication H9-201540). FIG. 8 is a diagram illustrating the structure of a conventional room pressure controlling system. The room pressure controlling system comprises: a fume hood 101 that is disposed within the room 100; a local exhaust duct 102 that is connected to the fume hood 101; a supply air duct 103 for supplying supply air to the room 100; a common exhaust duct 104 for the air of the room 100; a local exhaust air valve EXV for regulating the airflow rate of the local exhaust duct 102; a supply air valve MAV for regulating the airflow rate of the supply air duct 103; a common exhaust air valve GEX for regulating the airflow rate of the common exhaust duct 105; a controller 105 for controlling the local exhaust air valve EXV; a controller 106 for controlling the supply air valve MAV; a controller 107 for controlling the common exhaust air valve GEX; and communication lines 108 for connecting together the various controllers 105-1, 106, and 107. The fume hood 101 is provided with a sash 111 that can be opened and closed, and a sash sensor 112 for detecting the degree of opening of the sash 111.
In this type of room pressure controlling system, in order to maintain the pressure of the room 100 at the setting value, the degrees of opening of the supply air valve MAV and the common exhaust air valve GEX, and of the local exhaust air valve EXV, are controlled so that the supply airflow rate of the supply air duct 103, the exhaust airflow rate of the common exhaust duct 104, and the local exhaust airflow rate of the local exhaust duct 102 will satisfy the relationship of “Supply airflow rate=common exhaust airflow rate+local exhaust airflow rate+offset airflow rate.” Moreover, in recent years a PCV (pressure control valve) function, for performing stabilized pressure control by adjusting the degree of opening of the PCV valve based on a pressure differential by measuring the pressure differential between the inside and the outside of the room has been added as well. This PCV function is achieved through causing the room pressure controlling operations to be performed as well in addition to the actual functions of either the supply air valve MAV or the common exhaust air valve GEX.
Conventionally, the valve that has handled the PCV function has been established unchangingly at the time of system configuration, causing it to be operated more frequently then the other valves in order to perform the fine adjustment operations for the room pressure, which is problematic in that the frequency of actuation is higher, reducing the service life. When a valve that handles a PCV function fails, it tends to cause large failures in the room pressure control.
The present invention was created in order to solve the problems set forth above, and the object thereof is to provide a room pressure controlling system able to extend the service life of a valve that is provided with a PCV function and to be able to extend the up-time, and the time between maintenance.